1. Field of the Invention
The present invention relates generally to methods of and systems for computer aided diagnosis or screening of digital mammogram images. In its more particular aspects, in relates to identification of spots that may correspond suspect areas in a manner which in order to reduce the incidence of false positives includes a comparison of mammogram images of the same breast taken from different viewing or projection directions.
2. Description of the Related Art
A method of this general type is known from U.S. Pat. No. 5,133,020, particularly FIGS. 15(a) and (b) thereof and the associated discussion which describes identifying an "island" in one view of a breast as a false positive when there is no corresponding "island" at the same depth in a view of the same breast from a different viewing direction. The depth appears to be determined in each view perpendicular to a line in a mammogram coordinate direction which is tangent to a detected skinline.
Typically mammogram studies are taken from at least two viewing directions selected from the head-to-toe viewing direction known as cranio-caudal (CC), the side-to-side viewing directions known as medio-lateral (ML) or lateral-medial (ML) and the viewing direction which is generally at a 45.degree. angle between head-to-toe and side-to-side known as oblique (OB).
Early detection of breast cancer, the second most frequently occurring cancer in women, can significantly increase the patient's chances of survival. Such early detection requires periodic screening mammograms which are read by radiologists or mammographers for the presence of masses (also referred to as lesions or nodules) and clusters of microcalcifications as signs of malignancy. Both the number of mammograms to be interpreted and the difficulty of identifying these not easily recognizable signs of malignancy obscured by intensity gradations due to other tissue motivate developments in Computer-Aided Diagnosis of Mammograms (CADM) to at least automatically mark or enhance features of interest, including suspect areas, in images displayed on a monitor or printed on film or other media for visual interpretation by the mammographer.
Digital mammograms suitable for computer-aided diagnosis may be obtained by scanning film taken by conventional X-ray mammography or by utilizing other X-ray detector types that produce electronic image signals that may be directly digitized without the necessity of producing a film intermediate. These detector types include X-ray image intensifier/camera chain, photostimuable phosphor plate/laser readout (see U.S. Pat. No. 4,236,078), and selenium plate/electrometer readout technologies. Such technologies are progressing in their spatial resolution and contrast sensitivities achieved and the latter two, particularly, may soon find widespread use for mammographic applications.
One of the early steps in a CADM system is to segment a mammogram image into foreground (corresponding to the breast) and background (corresponding to the external surround of the breast). This segmentation reduces the amount of further processing because extraneous pixels belonging to the background are removed from further consideration. Also, the boundary contour or border between the foreground and the background, theoretically at the skinline, is ascertained. While the nipple is an important landmark, unless the nipple was marked by a metal bead, it is very difficult to detect in the mammogram. Often, it is not visualized in profile due to patient positioning or rolling over of the skin.
Next, a search for masses within the area segmented as corresponding to the breast may be accomplished by analyzing the size and shape of spots, sometimes referred to as "blobs" or "islands", that are discriminated by thresholding the mammogram at one or a few intensity levels. For example, in U.S. Pat. No. 5,212,637, a search for masses in different intensity ranges utilizes a calculated initial threshold value which threshold value is incremented no more than three times. "Blobs" produced by thresholding the mammogram at the initial or at an incremented threshold value, which correspond to regions having a sufficient prominence in intensity with respect to their immediate surround are classified as "potentially malignant" based on their size and shape, i.e. area, circularity, and eccentricity.
In U.S. Pat. No. 4,907,156, "islands" are produced by thresholding a difference image determined from a digital X-ray image. The threshold level is successively decreased which, in general, causes the islands to grow. At each threshold level, the shape and size of the islands are analyzed. An island is classified as a nodule based upon its effective diameter and circularity.
The aforementioned methods using a single mammogram view may yield an unacceptable number of false positive results, particularly when attempting to increase their sensitivity to a degree that false negatives are virtually eliminated. While there is the potential to use information from two different views of the same breast to reduce or eliminate these false positives as suggested in the aforementioned U.S. Pat. No. 5,133,020, in practice it is difficult to correlate positions in different views because the skinline alone is not a sufficient reference to determine if suspect masses in both views correspond to each other in position. Further, if a plurality of threshold values are used to discriminate spots in each view, the prior art does not suggest how any overlapping spots in each view should be compared.